//执行上下文的角度，var/let 融合
function foo(){
 var a = 1;
 let b = 2;
 {
    let b = 3;//维护一个栈
    var c = 4;
    let d = 5;
    console(a);
    console(b);
 }
 console.log(b);
 console.log(c);
 console.log(d);
}
foo();